Refrigerant systems are known to utilize refrigerant circulating throughout a closed-loop circuit to condition a secondary fluid. Typically, a refrigerant system includes a compressor for compressing the refrigerant, and delivering the refrigerant to a downstream heat exchanger. Refrigerant from that downstream heat exchanger passes through an expansion device, and then to an evaporator. In traditional refrigerant systems, the expansion device is a fixed area restriction or a valve that may be controlled such that the amount of expansion is tailored to achieve desired characteristics in operation of the refrigerant system.
In some advanced refrigerant systems, the work which is available from the expansion process of the refrigerant is utilized to drive or assist in driving at least one component within the refrigerant system.
In one known refrigerant system configuration, a secondary compressor operates in parallel with a main compressor. This secondary compressor compresses a portion of the refrigerant circulated throughout the refrigerant system. The secondary compressor is driven by the expander, with the expander operating much like a turbine, to receive the compressed refrigerant, and expand that refrigerant to a lower pressure and temperature. The work from this expansion process is utilized to drive the secondary compressor. This known combination of a compressor and an expander, located on the same shaft, is called an expresser. The use of the expresser is known in the industry, where the expander drives or assists in driving the corresponding compressor. The refrigerant exiting a heat rejection heat exchanger enters the expander, and then is expanded to a lower pressure and temperature. A two-phase flow exiting the expander enters the evaporator. The work extracted from the expansion process in the expander is used to drive the secondary compressor that is quite often located on the same shaft as the expander. In addition to extracting useful work from the expansion process, the refrigerant passing through the expander acquires a higher cooling thermodynamic potential, as it expands through the expander, since it follows a more efficient isentropic process. The use of the expresser technology is especially expected to grow in CO2 applications, where the potential for the expansion energy recovery is higher than for the conventional refrigerants.
One of the disadvantages of positioning the expander and the associated compressor into a closely coupled mechanical engagement, such as locating them on the same shaft, is that the expander speed is not actively controlled. In other words, the expander will settle at a speed at which the power extracted by the expander from the refrigerant expansion process is roughly equal to and is balanced by the power delivered to the compressor. Since the expander speed cannot be actively controlled, the expansion process through the expander is typically not optimal. If the expansion process is not optimal, then the amount of refrigerant delivered to the evaporator, and its thermodynamic state, cannot be precisely controlled. If a delivered amount of refrigerant cannot be adjusted, it may result, for instance, in less than optimal gas cooler pressure, in transcritical applications, and/or undesirable conditions at the compressor entrance.
In other words, to optimize the expansion process for given operating and environmental conditions, such as gas cooler pressure, suction superheat, etc., flexibility in varying the expander speed must be provided. One way to enhance the control of the expander is to install an expansion valve that is located in series with the expander. However, the expansion valve would reduce/limit the amount of the work extracted from the expansion process by the expander. This reduction would occur, as part of the expansion process would take place in the expansion valve, and not in the expander. Therefore, a need exists to optimize the expresser operation.